Check the Object Size
is not clear imho: do you wish to know the volume of the mesh, or the surface area, or the UVmap width/height?

I guess you perhaps need help from the (default bundled) 3d print addon:



this is what you get if you're using the default unit: "blender unit".

But you can change it to a custom unit, like this:



and get the area or volume calculated in the same unit (depending on your model size).

If you can't see this addon on the toolbox's side, you just need to enable it in user preferences.